1. Field of the Invention
The present invention relates to a method of detecting the status of the battery of an implantable heart stimulator, and to a battery status detecting circuit for an implantable heart stimulator.
2. Description of the Prior Art
It is of utmost importance to obtain reliable information about the remaining capacity or remaining charge of batteries used in implantable devices such as heart stimulators. From this information remaining operation time of the device can be determined and this enables the physician to plan for replacement of the battery and/or the heart stimulator at the appropriate time. Several techniques have therefore been used for monitoring the battery depletion and determining the remaining battery capacity.
One of the most commonly used methods is to monitor the internal impedance of the battery. This method is also considered to be a reliable way of determining the remaining capacity of a battery. However, this technique is marred by inconveniences. Thus, for a new battery the internal impedance has a low, substantially constant value for a comparatively long time and during this time it is difficult to perform reliable measurements of the small changes which may appear in the impedance. Measurement of the internal battery impedance will consequently not provide reliable information during a comparatively long period of the early phase of battery depletion. In this phase the internal battery impedance cannot be used for determining the battery status and the remaining operation time of the heart stimulator, which is a disadvantage since it is often desirable to be able to predict already at a comparatively early stage the duration of the remaining operation time.
U.S. Pat. No. 5,370,668 discloses an implantable medical device in which internal battery impedance measurements are combined with periodic assessments of the loaded terminal voltage of the battery to obtain an elective replacement indication for when the battery depletion reaches such a level that replacement will soon be needed. The technique used in this device is adapted particularly for rejecting transients in the battery""s demand as criteria for triggering an elective replacement indication.
From a theoretical point of view the ideal way of determining the remaining capacity of a battery would be measurement of the charge drawn from the battery. Such techniques are proposed in e.g. U.S. Pat. Nos. 4,715,381 and 5,769,873. In U.S. Pat. No. 4,515,381 a battery test circuit for a heart stimulator is described for identifying the consumed charge from the number of stimulation pulses emitted and from the charge related to each pulse. Other losses of current, like e.g. leakage currents, are not considered. The true remaining battery capacity could then be less than the estimated remaining capacity and consequently the remaining operation time could be overestimated. U.S. Pat. No. 5,769,873 discloses an improvement in so far that the electrical current drawn from the battery is measured and integrated. It has, however, appeared that measurement of actually depleted charge in this way will be similar to the measurements above, since spread in individual battery capacity and possible leakage currents in supply voltage stabilizing capacitors etc. are not taken into account.
An object of the present invention is to remedy the above discussed deficiencies of the prior art and to provide a battery monitoring method and circuit which allow a new way of obtaining reliable information about remaining operation time of the battery of an implantable heart stimulator all the time from the early depletion phase of the battery.
The above object is achieved in accordance with the principles of the present invention in a battery status detection circuit and method wherein battery impedance is measured and an impedance-based value of the remaining capacity of the battery is determined from a detected impedance increase, and wherein an analysis of the battery impedance increase is performed to determine whether the battery impedance is a reliable indicator of the remaining battery capacity and, if not, the total charge depleted from the battery is measured and a charge depletion-based value of the remaining capacity of the battery is determined.
Thus in the present invention measurements of the internal battery impedance, comprising the steps of measuring the impedance of the battery, detecting an increased value of the measured impedance, and determining an impedance based value of the remaining capacity of the battery from the detected impedance increase, are combined with measurements of the total charge depleted from the battery, comprising the steps of measuring the total charge depleted from the battery, and determining a charge depletion based value of the remaining capacity of the battery. The battery status detecting circuit according to the invention includes a first measurement stage for measuring the impedance of the battery, a detection unit for detecting an increased value of the measured impedance, a first determination unit for determining an impedance based value of the remaining capacity of the battery from the detected impedance increase, a second measurement unit for measuring the total charge depleted from the battery, and a second determination unit for determining a charge depletion based value of the remaining battery capacity.
Improved battery status information is obtained in this way during the entire battery discharge cycle, and a reliable estimate of the remaining operation time can be obtained at any time of the cycle. Thus, with the present invention the remaining battery capacity is determined by two separate measurements, and if the internal battery impedance measurement is assumed to give a reliable resultxe2x80x94in the later phase of the battery discharge cyclexe2x80x94this measurement result will normally have priority over the measurement of depleted charge.
In an embodiment of the method according to the invention the depleted charged is measured during the whole lifetime of the battery, since this measurement can be used during the whole battery discharge cycle for estimating the remaining operating time of the battery. The impedance measurements can be started when the measured charge depleted from the battery amounts to a predetermined limit value, viz, in a later phase of the battery discharge cycle, or when other criteria relating to the remaining charge in the battery have been met, since measurements of the internal battery impedance cannot suitably be used for determining remaining operation time during the early phase of the battery discharge cycle as discussed above. Only in the later phase of this battery discharge cycle double security from two independent measurements are obtained. However, a reliable and precise estimate of the remaining operation time is, of course, most important in this later phase of the battery life time. The step of determining a charge depletion based value of the remaining battery capacity is performed only if no increased impedance is detected, suitable for a reliable impedance based determination of the remaining battery capacity, because as soon as a reliable impedance based determination of the remaining battery capacity is obtained, this result is normally considered to be more reliable and has priority over the results obtained from measurements of depleted charge. In addition to the charge-depletion value discussed above, the impedance measurement means could thus be adapted to start impedance evaluation when the measured impedance has reached a predetermined limit value, for instance a predetermined absolute value of the impedance or a predetermined slope of the impedance-charge curve. As mentioned above, the internal battery impedance is substantially constant during the early phase of the battery discharge cycle and said predetermined limit values is therefore selected to be in the vicinity of that point where the start of a significant increase in the internal impedance is expected.
In another embodiment of the method according to the invention the impedance based and the charge depletion based values of remaining battery capacity are compared and a discrepancy signal is produced if the difference between the two values of the remaining battery capacity exceeds a predetermined limit. Such a difference in a phase of the battery discharge cycle where the two measurements are supposed to give substantially the same result indicates that there is some kind of problem with the implanted stimulator.
Thus, the user can at all times obtain reliable information about remaining longevity for different operating conditions. If the remaining capacity as determined by the two methods differs significantly this indicates that there is might be an early battery depletion caused e.g. by normal spread in battery capacity or by a fault either in the electronics of the stimulator or in battery.
In an embodiment of the circuit according to the invention the charge depletion measurement means is adapted to measure the time integral of the current supplied to the stimulator electronics and the total charge contained in delivered stimulation pulses. Thus the total measured charge drawn from the battery consists of two components, i.e. the current to the stimulator electronics and the delivered stimulation pulses.